Memory devices have become increasingly popular in various electronic devices. In a memory device, a plurality of groups of memory cells (e.g., memory strings) are coupled to corresponding ones of a plurality of metal bit lines that are pre-charged to a predetermined voltage level. Data stored in a memory cell in a memory string can be sensed by applying a select voltage to the memory cell and sensing the current on the corresponding metal bit line. However, due to the increasing density of memory cells, smaller cell current, and large loadings of the metal bit lines, the time required for pre-charging the metal bit lines become longer.